Topics Covered

Background

Photoluminescence is a well-established, non-contact, nondestructive technique used in the development and process control of semiconductors, with room temperature PL wafer maps giving vital information on the uniformity of alloy composition, material quality and defects in substrates, epilayers and device structures.

RPM2000 Rapid Photoluminescence Mapper from Nanometrics

The RPM2000 has been designed specifically to obtain whole wafer PL maps in a mere fraction of the time previously associated with this sort of measurement, giving the ability to measure and assess wafers between production runs. The primary benefit is that rapid feedback and remedial action can be implemented should wafer parameters be out of specification, avoiding wasted production runs, thus saving time and costs. In addition, the speed of measurement makes incoming inspection and qualification of bought-in wafers a quick and easy task.

By using a unique design, the system achieves super fast mapping speeds without compromising spectral and spatial resolution. For example, a rapid check of integrated PL signal on a 2-inch wafer at 1mm spatial resolution can be performed in only 19 seconds. That's a 2026 point map in less than 20 seconds. Full spectral mapping, at the same spatial resolution, giving peak position, peak intensity, FWHM and integrated intensity would only take 25 seconds. In under 100 seconds, the RPM2000 can produce integrated maps of a 2-inch wafer at 0.2mm spatial resolution, a total of over 50,000 points, or spectral maps at 0.5mm spatial resolution giving 8,000 points.

The RPM2000 can also provide true image quality PL mapping at high spatial and spectral resolutions, giving the ability to examine hundreds of thousands of points on wafers up to 150mm diameter: It is possible to map at 0.1mm spatial resolution on wafers up to 100mm diameter and 0.2mm on wafers up to 150mm. For 150mm wafers a 1mm spatial resolution integrated map would take only 56 seconds and an image quality 0.2mm spatial resolution integrated map, corresponding to over 455,800 points would take only 8 minutes.

Hardware

The RPM2000 uses a custom designed R, È, Z stage which holds wafers of up to 150mm diameter and thicknesses up to 1mm. Temperature and laser power monitors are fitted as standard. The sample holder comprises two parts: the scanning stage and a quick and easy to change wafer chuck, chosen to match the wafer diameter. The wafer is held by vacuum allowing small samples of irregular shape to be accommodated. The PL map is generated by moving the wafer while the laser excitation beam remains stationary. By using array detectors, all wavelengths can be acquired simultaneously.

System Configurations

The RPM2000 can be configured to cover a wide spectral range, defined by the choice of gratings, excitation lasers, detectors and order sorting filters. Our scientists can advise on custom configurations.

Excitation and Detection

Up to two internally mounted excitation lasers and one external optical fiber coupled excitation laser can be accommodated. Excitation laser beams hit the sample surface at an angle in order to provide spatial filtering of the laser light. The beam is normally focused to a spot diameter of approximately 100µm and, typically, the laser power is kept low enough to eliminate any change of spectra. The PL produced by the sample is collected by reflective optics, coupled to a 0.3m monochromator, giving excellent throughput across the spectral range. Up to three computer controlled gratings, seven filters and two array detectors may be installed.

A third detector is used for measuring integrated PL signals. Lasers, detectors, filters and gratings are all selectable from the software without the need to reconfigure the hardware. A reflectivity option is also available for spectral systems to determine film thickness and for Bragg reflector and VCSEL characterization.

The RPM Software

The RPM2000 application has been developed over many years and is robust and versatile. All required control and analysis functions are directly available as part of the application. Functions are presented in a logical and intuitive manner. The application comes complete with a Recipe Editor and a comprehensive Online Help Manual.

High-Spatial Resolution Mapping

The RPM2000 can collect up to 200 data points per second in full spectral mode with on-board Digital Signal Processing (DSP) or an astonishing 2000 points per second in spectral integrating mode. This enables very high spatial resolution "wafer images" to be collected. These images can often reveal small growth defects that would otherwise go undetected. High spatial resolution in an r-theta scan brings another advantage, The ability to image the edge region of a wafer in great detail.

The RPM2000 can collect up to 2000 data points per second in non-spectral mode. The image reveals small growth defects that are undetectable when the wafer is scanned at low spatial resolution.

Fourier Filtering for GaN LED's

Raw PL data from a GaN MQW LED is often distorted by the presence of interference fringes. A sophisticated Fourier Filtering software algorithm mathematically removes the effect of the fringes - revealing the true PL spectrum.

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